Oil premix burner

ABSTRACT

An oil premix burner, having a burning element, a burner surface on the outer lateral surface, a distribution chamber below the burner surface, a central oil injection device, a combustion air duct connected to a fan situated upstream, and an air heat exchanger for preheating the combustion air in the area of the burner surface. The combustion quality and the robustness are optimized in an oil premix burner, in particular with respect to the modulation capability. It is provided that the air heat exchanger is composed of at least one double tube element including an inner tube, having inline throughflow, and an outer tube concentric to the inner tube. The air heat exchanger is situated above the burner surface and is composed of multiple parallel throughflow double tube elements symmetrically distributed over the circumference and which extend axis-parallel to the burning element at least over a portion of its length.

THE FIELD OF THE INVENTION

The present invention relates to an oil premix burner.

BACKGROUND INFORMATION

These types of oil premix burners have a cylindrical burning elementwith a burner surface on the outer lateral surface. Generally, there isa central oil injection device around a distribution chamber for thegenerated oil mist so that this reaches the burner surface evenly frominside. Combustion air reaches the distribution chamber via at least oneinlet opening in the area of the oil injection device which may be aninjector or an oil nozzle, which is fastened to a nozzle fitting havingan oil preheater. Upstream from the oil injection device is a combustionair duct having a connection to a fan.

The so-called premix burners have been accepted in particular for gasburners which are operated in a modulated manner, thus, in which thecapacity range maybe controlled between a small portion of the maximumcapacity and the maximum capacity. In these burners, the fuel and thecombustion air required for its combustion are locally separated fromthe flame, mixed and then burned. For this purpose a cylindrical burningelement is usually used as a flame holder which is permeable to thefuel/air mixture and which surrounds an inner space to which thefuel/air mixture is supplied. Using this technology, it is possible toimplement small flames having good exhaust gas values, relatively smallflame holders and large modulation ranges.

The basic principle of an oil premix burner is the mixing of finelyatomized oil with preheated air. By using this principle it is possiblethat, for example, a so-called cold flame is formed so that the fuelmaybe burned over a surface. However, surface burning is also possiblewithout a cold flame. In general, the heating of air is relativelydifficult due to the low heat capacity, because either an effective heattransfer via a wall to the air flow and/or high wall temperatures arenecessary for this purpose.

For example, an oil premix burner is discussed in DE 10 2006 000 174 A1.Here, use is made of the fact that hot air vaporizes the oil and thus agas mixture may be formed, which, as in the case of a gas premix burner,passes through the flame holder and burns away. By admixing exhaust gasto combustion air for air preheating, the formation of nitrogen oxidesis additionally reduced and in this way the starting behavior shouldalso be improved because the operating point is reached relativelyquickly. For heat absorption, the combustion air is guided through aguiding system, which projects at least in sections over a burnersurface, and is distanced from the burner flame and is accelerated in anozzle in the guiding system so that the exhaust gas may be drawn fromthe combustion chamber and mixed with the air.

German patent document DE 26 43 293 A1 also discusses that such a ductfor the combustion air which is situated as a cylinder around the reararea of a burning rod or burning element. Since a certain minimumburning element diameter is necessary for the mixing and the combustionreaction, this configuration may result in a relatively large combustionchamber in contrast to known configurations.

Furthermore, a system is discussed in DE 21 07 514 A for heating usinginfrared radiation, and from U.S. Pat. No. 1,082,576 A an oil burner isdiscussed having closed, continuous ducts situated flat below the burnersurface for preheating the air. From these uninterrupted ducts, in bothcases the preheated air is guided back to the center with the fuelsupply and there mixed with fuel. From this center the mixture entersthe combustion zone without again coming into the vicinity of the airflow path used for preheating.

SUMMARY OF THE INVENTION

An object of the exemplary embodiments and/or exemplary methods of thepresent invention is thus to optimize the combustion quality androbustness in an oil premix burner in particular in view of themodulation capability.

According to the exemplary embodiments and/or exemplary methods of thepresent invention, this is achieved with the features described herein.Further refinements may be derived from the further description herein.

The oil premix burner is characterized in that the air heat exchanger iscomposed of at least one double tube element including an inner tubehaving an inline throughflow and an outer tube concentric thereto. Inthis case, the air heat exchanger is situated above the burner surfaceand is composed of multiple parallel throughflow double tube elementssymmetrically distributed over the circumference and extendingaxis-parallel to the burning element at least over a portion of itslength.

Each double tube element has on the front side of the free end adeflection zone as a connection between the inner tube and an annularspace formed between the inner tube and the outer tube.

Thus, each double tube element is advantageously connected at its entryend of the inner tube to the combustion air duct.

There the combustion air enters and, after leaving the inner tube,arrives in a deflection zone at the front side of the free end in theannular space formed between the inner tube and the outer tube. Here,the already preheated air flows back in the direction of the oilinjection device near the front side and picks up additional heat. Thereit enters the distribution chamber in the area around the oil injectiondevice as preheated combustion air.

In one specific embodiment, in a double tube element, the inner tubeprojects beyond the outer tube in the longitudinal direction at the endnear the oil injection device, so that it passes through the outlet areaof the annular gap (space) which is connected to the distributionchamber and reaches through a boundary wall up into the combustion airduct.

In one further specific embodiment, the inner tube and/or the outer tubehas/have turbulence-generating surfaces, a profiling and/orsurface-enlarging elements in order to improve the heat transfer and inparticular to break up the flow near the walls. For this purpose, theinner tube and/or the outer tube is/are provided with profiles and/orbeads for turbulence-generation, which may run spirally on the tubesurface. It is particularly advantageous when at least the inner tube isprovided on its outer side with means for influencing the flow in thesurrounding annular gap (space).

The diameters of the inner tube and the outer tube may be dimensioned insuch a way that the free cross section in the course of the flow path ofthe combustion air remains at least the same or increases. In this wayan adaptation of the volume which increases with the air temperature isachieved. On the other hand, the flow velocity and thus the heatabsorption capability and also the thermal load of the outer surface areinfluenceable in a targeted manner in the respective duct areas via thecross-section formation.

In one further specific embodiment, the inner tube may also be providedwith multiple openings in its housing. These permit a direct flow intothe surrounding annular gap (space), create local turbulences andcooling effects and, in cross section, may each be ten times smallerthan the internal diameter of the inner tube.

In addition, the exemplary embodiments and/or exemplary methods of thepresent invention provide that the inner tube and the outer tube aremutually supported at least at selected points. This takes place inparticular in the deflection zone at the front side of the free endand/or via fittings and/or a special configuration of the tube wall, inthat the deflection zone, for example, is provided with web-likeprojections, bumps, small chamfers in the end area or local impressions.

Using the measures according to the exemplary embodiments and/orexemplary methods of the present invention, both the combustion qualityand the robustness are improved for oil premix burners. The systemaccording to the present invention having the integrated air heatexchanger is advantageous in particular for modulated operation. Thisapplies to burners having a cylindrical burning element as well as tosurface burners having a horizontal burner surface and the configurationaccording to the present invention. The flame burns on or in the burnersurface so that above it energy is extracted from the flame and used forpreheating the combustion air. In this way electrical preheating isdispensed with during the stationary burner operating state. This has apositive effect on the energy balance of the burner.

An important advantage of the integrated air preheating functionaccording to the present invention is that with increasing burnersurface, for example in the case of a burning element for largercapacity ranges, the heat exchanger surface is expandable to the sameextent in that simply longer double tube elements are used. In this waythe sought air temperature remains constant. Each tube is definedaccording to its specific capacity density and the number of tubes as afunction of the overall capacity. Except at the double tube elements ofthe air heat exchanger, no structural modifications are necessary at theperiphery or other burner components for demonstrating different burnerperformances. The other burner configuration thus remains just the sameas are the demands on the surrounding combustion chamber.

Using the air heat exchanger according to the present invention,combustion air should be provided at all burner operating points havinga minimum temperature of approximately 350° C. Here it must be notedthat under observance of the maximum permissible surface temperature atthe air heat exchanger, the length of tube elements of an availableconfiguration, i.e., in the form of smooth, U-shaped heat exchangercoils, as well as the number of the tubes cannot be increasedarbitrarily. In addition to the limits in the observance of the maximumbending radius, the material temperatures also continue to rise whenlonger conventional tubes are used, entailing the risk of deflection.

In contrast, according to the exemplary embodiments and/or exemplarymethods of the present invention, a longitudinal expansion due tothermal loading is possible since each double tube element of the airheat exchanger is only fixed at one end. Longer lengths correspondinglyadapted to a burning element or cantilevers of double tube elements maybe implemented via mutual support of the inner tube and the outer tube.

The use of profiled tubes which favor the heat transfer and which ensurea low material-preserving surface temperature becomes possible only withthe aid of the present invention.

In particular the inner tube may act as turbulence exciter or a flowbreaker. In general, by using the double tube element according to theexemplary embodiments and/or exemplary methods of the present invention,a higher packing density on a pitch circle in the area of the burnersurface may be achieved in contrast to the related art. In addition,these double tube elements have advantages with regard to pressure lossand scalability. In principle, according to the exemplary embodimentsand/or exemplary methods of the present invention, a counter-flow heatexchanger is implemented by using a double tube element since the airflow in the two ducts moves in opposite directions.

The drawing shows an exemplary embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an oil premix burner in longitudinal section.

FIG. 2 shows a longitudinal section through a part of the air heatexchanger.

DETAILED DESCRIPTION

The oil premix burner is composed essentially of a cylindrical burningelement 1 having a burner surface 2 on the outer lateral surface, adistribution chamber 3 below burner surface 2, a central oil injectiondevice 4 and a combustion air duct 5 connected to a fan situatedupstream.

For preheating the combustion air in the area above burner surface 2,burning element 1 has an air heat exchanger composed of multipleparallel throughflow double tube elements 6 symmetrically distributedover the circumference and including an inner tube 7, having an inlinethroughflow, and an outer tube 8 concentric thereto.

Each double tube element 6 is closed at the front side of the free endwith a cap 9 so that a flow-advantageous deflection zone 10 is aconnection between inner tube 7 and an annular space 11 formed betweenit and outer tube 8. From combustion air duct 5, combustion air entersinner tube 7 of each double tube element 6 and flows via deflection zone10 into annular space 11. Herein it flows back in the direction of oilinjection device 4 and enters distribution chamber 3.

Both inner and outer tubes 7 and 8 have turbulence generating surfacesand are equipped with a profiling 12 shown as spirally running beads onthe tube surfaces.

1-11. (canceled)
 12. An oil premix burner, comprising: a burningelement, a burner surface on the outer lateral surface, a distributionchamber below the burner surface; a central oil injection device; acombustion air duct connected to a fan situated upstream; and an airheat exchanger for preheating the combustion air in the area of theburner surface, wherein the air heat exchanger includes at least onedouble tube element including an inner tube, having an inlinethroughflow, and an outer tube concentric thereto.
 13. The oil premixburner of claim 11, wherein the air heat exchanger is situated above theburner surface and includes multiple parallel throughflow double tubeelements symmetrically distributed over the circumference and extendingaxis-parallel to the burning element at least over a portion of itslength.
 14. The oil premix burner of claim 11, wherein each double tubeelement has a deflection zone on the front side of the free end as aconnection between the inner tube and an annular space formed betweenthe inner tube and the outer tube.
 15. The oil premix burner of claim11, wherein each double tube element is connected with the entry side ofthe inner tube to the combustion air duct, wherein the combustion air,after leaving the inner tube, arrives in a deflection zone on the frontside of the free end and flows into the annular space formed between theinner tube and the outer tube and flows back in the direction of thefront side near the oil injection device and there enters thedistribution chamber in the area around the oil injection device aspreheated combustion air.
 16. The oil premix burner of claim 11, whereinin a double tube element, the inner tube projects at the end near theoil injection device beyond the outer tube in the longitudinal directionso that it passes through the outlet area of the annular space connectedto the distribution chamber and reaches through a boundary wall up intothe combustion air duct.
 17. The oil premix burner of claim 11, whereinthe inner tube and/or the outer tube has/have turbulence generatingsurfaces, a profiling and/or surface enlarging elements.
 18. The oilpremix burner of claim 11, wherein the inner tube and/or the outer tubeare provided with profilings and/or beads for turbulence generation,which run spirally on the tube surface.
 19. The oil premix burner ofclaim 11, wherein at least the inner tube on its outer side has anarrangement for influencing the flow in the surrounding annular space.20. The oil premix burner of claim 11, wherein the diameters of theinner tube and the outer tube are dimensioned so that the free crosssection in the course of the flow path of the combustion air remains atleast the same or increases.
 21. The oil premix burner of claim 11,wherein the inner tube has multiple openings in its housing which permita direct flow into the surrounding annular space and generate localturbulences and cooling effects in the annular space and, in crosssection, are at least ten times smaller than the internal diameter ofthe inner tube.
 22. The oil premix burner of claim 11, wherein the innertube and the outer tube are mutually supported at least at selectedpoints, this taking place in particular in the deflection zone on thefront side of the free end and/or via fittings and/or a special designof the tube wall.